On an exchange with no LLU available then for a moderate user like your mum I very much doubt you can beat Plusnet's current deal as long as she is prepared to sign a 1 year contract.
This is £19.50 per month for both phone line rental and broadband with free evening and weekend calls and a 10Gb download allowance on the broadband. But the first 3 months are only £5.50 per month and there is either a fee 4 port router included or £15 cashback if she doesn't need that. Due to the first 3 months at the lower price then if you also take the £15 cashback this really means the broadband is only costing £5 per month above the BT line rental of £10.25 or so that she would clearly be paying anyway.
Basically this is TalkTalk/Tiscali type combined broadband and phone rental pricing on a non LLU exchange (where they don't offer that kind of pricing) but with a supplier (Plusnet) with English call centres, fairly helpful staff and a good reputation for reliability and support.
Note that this deal with Plusnet ends on 28th January so she would need to get a MAC code from Sky by Wednesday to take advantage.
See
www.plus.net/residential/broadband/broadband_with_homephone_us.shtml for more information.
If she is on a non LLU exchange then Sky's Broadband Connect is just a normal BT Wholesale internet connection and there will be no fee for terminating or moving elsewhere as long as as she has been with Sky for broadband for at least 12 months.
Edited by deleted (Mon 26-Jan-09 11:02:16)